ABSTRACT
Time-dependent reductions infracture conductivity have been implied in instances where post-treatment well test data have been available and analyzed. This study evaluates the importance of fatigue failure in a cyclically stressed proppant pack as a contributing factor to fracture conductivity impairment. Permeabilities and pack widths for common mesh sizes of a sand and a ceramic proppant, using formation core plates of varying hardness, were measured after repeated closure stress cycles. Interactions between proppant and formation materials were examined in the SEM in an attempt to correlate fatigue-induced damage with known lithological classifications, and with proppant particle microstructures.
Under the action of repeated stresses, such as in the production/shut-in cycles of some hydraulically fractured wells, fatigue failure in the propped fracture occurs within the proppant particle as well as on the fracture face. The data obtained in these experiments show that both proppant permeability and fracture width gradually decrease with increasing number of cycles, as the proppant and adjacent formation break down, and the supporting structure of the propped fracture progressively weakens. The ceramic proppant exhibited better overall resistance to fatigue failure than the natural sand did. Experiments run with low hardness core plates showed increased sensitivity to repeated stresses, reflected by a greater degree of fracture width reduction and flow channel obstruction. The effect of fatigue-induced fracture width reduction is expected to increase with decreasing proppant concentration.
